The problem with urea is that at alkaline pH it decomposes into ammonia
and cyanate (the reversal of Woehler's urea synthesis back in the 19th
century). The cyanate ion can react with amino groups (esp. lysine) and
form a carbamoylate (R-NH-CONH2) which alters the properties of the
amino group quite a lot (pK, reactivity).
So you should definetly prepare fresh solutions for buffer B (pH 8), the
one with the lower pH can be stored for some days in the frige (low
temperature slows down decompostion) I would guess.
Hope this helps,
Stefan Walter, Ph.D.
Yale University School of Medicine